The invention relates to nematic liquid crystal mixtures for use in electro-optical devices on the basis of the guest-host effect in order to modulate the transmitted or incident light particularly for the colored reproduction of numerals, symbols and images.
Nematic liquid crystals are known which can be reoriented in electrical fields due to their dielectric anisotropy. If the nematic liquid crystals are placed in suitable electrical cells it is possible to provide devices for modulation of the transmittent or incident light and, as above stated, for the colored rendition of numerals, symbols and images.
The technical effect of these electro-optical structural elements, however, may be based on different technical mechanisms. If nematic crystals are used with a twisted structure, two polarization filters are necessary, and only the black and white rendition of symbols is possible (M. Schadt, W. Helfrich: Applied Physics Letters 18, 127 (1971)).
If, however, cells are used which operate on the basis of the guest-host effect, only one polarizer is necessary, and a colored reproduction is possible (G. H. Heilmeier, L. A. Zanoni: Applied Physics Letters 13, 91 (1968). However, with crystals adapted for the guest-host effect it is necessary to add to the liquid crystal a dichroic dyestuff which is oriented simultaneously with the liquid crystal and can be switched due to the action of the electrical field between an absorption position and a light transmitting position.
The dyestuffs heretofore known for this purpose were usually chemical compositions with azo groups which either were slightly soluble in the liquid crystals or over an extended period of time underwent changes under the action of visible and ultraviolet light or also exhibited a dichroism which was too weak. Thus, these electro-optical structural elements were not useful for operation over long periods of time because the dyestuffs crystallized out or because decomposition at longer times of operation resulted in a decrease of the possible color contrast.
It is therefore an object of the invention to provide for electro-optical structural elements on the basis of the guest-host effect for the modulation of light and colored rendition of images, etc. which also have a strong color contrast for extended periods of operation and are devoid of the shortcomings of the prior art products.
The purpose of the invention is therefore the provision of a mixture of a dyestuff, or provision of a specific dyestuff which can be mixed, with a nematic liquid crystal and which has a high chemical and thermal stability and a pronounced dichroism in the visible spectrum range and is therefore suited for electro-optical elements intended for long periods of operation.